1. Field of the Invention
The present invention relates to a focus servo circuit and method for focusing a beam used to reproduce data from a disc, and in particular, to an improved focus servo apparatus for a multi-layer disc which is capable of carrying out a stable focus servo operation using a level holder and a focus jump function.
2. Description of the Prior Art
FIG. 1 shows a focus servo unit for a single-layer disc according to the conventional art, which includes a focus servo pulling-in unit 1 that outputs a driving signal for pulling in an objective lens towards a data surface of a disc, a main focus servo unit 2 that outputs a driving signal for adjusting a focusing of the objective lens at a pulled-in position, a switch 3 for selecting among the driving signals output from the focus servo pulling-in unit 1 and the focus servo main unit, a focus coil 4 for pulling in the objective lens in accordance with a driving signal selected by switch 3, and a focus error detector 5 for generating a focus error signal FE by detecting a focus error signal (FE) from a light signal reflected from a disc signal surface and input through the focus coil 4 and for supplying the focus error signal (FE) to the main focus servo unit 2.
The operation of the focus servo apparatus for a single-layer disc according to the conventional art will not be described in detail.
When information is reproduced from a CD player having a single-layer disc, the focus servo pulling-in unit 1 outputs a DC (direct current) driving signal through the switch to the focus coil 4 for an interval (t2,t3) through the switch in a pulling-in ready condition` as shown in FIG. 3.
The focus coil 4 lowers the objected lens for an interval (t2) in accordance with the DC driving signal output from the focus servo pulling-in unit 1. Then, the focus coil 4 slowly raises the objective lens and fixes the focus of the pick-up onto the data surface of the disc for an interval (t3).
When the objective lens reaches the focus position, the switch 3 is switched to the main focus servo unit 2, resulting in the formation of a focus servo main loop (ML), and shutting off the output of the focus servo pulling-in unit 1.
The focus servo main loop (ML) is formed when the switch 3 is connected to the main focus servo unit 2. This loop is controlled based on the AC (alternating current) focus error signal (FE) indicating the extent of deviation between the focus of the pick-up and the date surface of the disc, where the focus error signal (FE) is varied in accordance with the up-and-down movement of the focus position of the pick-up.
When the objective lens reaches the accurate focus position, no focus error signal (FE) is formed in the focus servo main loop (ML). For that reason the driving signal is not output from the main focus servo unit 2 and, as a result, the objective lens is momentarily lowered at interval t4, as shown in FIG. 3.
Due to the momentary lowering of the objective lens, a light signal reflected from the data surface of the disc is input to the focus error detector 5 through the focus coil 4 to generate an AC focus error signal (FE). In response to the focus error signal (FE), the main focus servo unit 2 outputs a driving signal, arranges the objective lens to the focus position, and performs a reproducing operation of data.
But, as shown in FIG. 2A, when the focus position of the objective lens is set to a standard position thereof, the focus error signal (FE) becomes larger than when the focus position of the objective lens is close to the standard position thereof, and as a result, the precise focusing is impossible to achieve.
Furthermore, when there is one focus position, as in the focus servo apparatus for the single-layer disc, and the variation of the focus position is maintained within the range of an allowable error, the precise focusing is not influenced. However, when there are several focus positions and the focus of the pick-up should be varied from one layer to another layer as in the focus servo apparatus for multi-layer disc, a precise focusing is difficult to carry out due to a large variation of the focus error signal (FE).
Moreover, when the conventional focus servo apparatus for a single-layer disc pulls in the objective lens, it cannot prevent the objective lens from lowering momentarily. For that reason in multi-layer discs, the precise and stable focusing operation cannot be performed, in accordance with the focus error signal (FE) in which the focus position of the pick-up should vary for each layer.